unlink gave the error: cannot unlink `test_dir': Not a directory. And I've added the straces' output to the question.
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benMar 21 '12 at 15:02

Very strange: IMHO this means that the path contains at some point something which is not a directory as is touch h; unlink h/o. Did you try to specify the absolute path to rm / unlink? (I cannot explain things, it's just a try.)
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Andreas FlorathMar 21 '12 at 15:49

I managed to solve this, even though I don't fully understand how or what really went wrong.

Looking at Mystery files, "not a directory", I found someone with a similar problem. They were able delete the "mystery file" by ssh'ing into the machine that was doing the sharing and delete the file from there.

I don't have ssh privileges to that machine, but I can FTP. And, amazingly, I was able to delete the link.

I guess the problem was a combination of Samba and bad ln calls. Thanks for all your help getting this sorted.